Microelectronic assemblies having compliant layers

ABSTRACT

A microelectronic assembly includes a microelectronic element having a first surface including a central region and a peripheral region surrounding the central region, the microelectronic element including a plurality of contacts disposed in the central region. The microelectronic assembly also includes a compliant layer overlying the peripheral region of the first surface, the compliant layer having a bottom surface facing toward the first surface of the microelectronic element, a top surface facing upwardly away from the first surface of the microelectronic element and one or more edge surfaces extending between the top and bottom surfaces of the compliant layer. A plurality of flexible bond ribbons are disposed over the compliant layer so that the bond ribbons extend over the top surface and one or more of the edge surfaces and the bond ribbons electrically connect the contacts to conductive terminals overlying the top surface of the compliant layer.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patentapplication Ser. No. 10/107,094 filed Mar. 26, 2002, which is acontinuation of U.S. patent application Ser. No. 09/777,782, filed Feb.6, 2001, which is a continuation of U.S. patent application Ser. No.09/071,412, filed May 1, 1998, which is a continuation-in-part of U.S.patent application Ser. No. 08/739,303, filed Oct. 29, 1996, now U.S.Pat. No. 6,211,572, which, in turn, claims benefit of U.S. ProvisionalApplication No. 60/007,128, filed Oct. 31, 1995, the disclosures ofwhich are hereby incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates to semiconductor chip packaging.More particularly, the present invention relates to an improvedcompliant semiconductor package structure and methods for making thesame.

FIELD OF THE INVENTION

[0003] The present invention relates to semiconductor chip packaging.More particularly, the present invention relates to an improvedcompliant semiconductor package structure and methods for making thesame.

BACKGROUND OF THE INVENTION

[0004] Complex microelectronic devices such as modern semiconductorchips require numerous connections to other electronic components. Forexample, a complex microprocessor chip may require many hundreds ofconnections to external devices.

[0005] Semiconductor chips commonly have been connected to electricaltraces on mounting substrates by one of three methods: wire bonding,tape automated bonding, and flip-chip bonding. In wire bonding, the chipis positioned on a substrate with a bottom or back surface of the chipabutting the substrate and with the contact-bearing front or top surfaceof the chip facing upwardly, away from the substrate. Individual gold oraluminum wires are connected between the contacts on the chip and padson the substrate. In tape automated bonding a flexible dielectric tapewith a prefabricated array of leads thereon is positioned over the chipand substrate and the individual leads are bonded to the contacts on thechip and to pads on the substrate. In both wire bonding and conventionaltape automated bonding, the pads on the substrate are arranged outsideof the area covered by the chip, so that the wires or leads fan out fromthe chip to the surrounding pads. The area covered by the subassembly asa whole is considerably larger than the area covered by the chip. Thismakes the entire assembly substantially larger than it otherwise wouldbe. Because the speed with which a microelectronic assembly can operateis inversely related to its size, this presents a serious drawback.Moreover, the wire bonding and tape automated bonding approaches aregenerally most workable with chips having contacts disposed in rowsextending along the periphery of the chip. They generally do not lendthemselves to use with chips having contacts disposed in a so-calledarea array, i.e., a grid-like pattern covering all or a substantialportion of the chip front surface.

[0006] In the flip-chip mounting technique, the contact-bearing surfaceof the chip faces towards the substrate. Each contact on the chip isjoined by a solder bond to the corresponding pad on the substrate, as bypositioning solder balls on the substrate or chip, juxtaposing the chipwith the substrate in the front-facedown orientation and momentarilymelting or reflowing the solder. The flip-chip technique yields acompact assembly, which occupies an area of the substrate no larger thanthe area of the chip itself. However, flip-chip assemblies suffer fromsignificant problems with thermal stress. The solder bonds between thechip contacts and substrate are substantially rigid. Changes in the sizeof the chip and of the substrate due to thermal expansion andcontraction in service create substantial stresses in these rigid bonds,which in turn can lead to fatigue failure of the bonds. Moreover, it isdifficult to test the chip before attaching it to the substrate, andhence difficult to maintain the required outgoing quality level in thefinished assembly, particularly where the assembly includes numerouschips.

[0007] Numerous attempts have been made to solve the foregoing problem.Useful solutions are disclosed in commonly assigned U.S. Pat. Nos.5,148,265 and 5,148,266. Preferred embodiments of the structuresdisclosed in these patents incorporate flexible, sheet-like structuresreferred to as “interposers” or “chip carriers”. The preferred chipcarriers have a plurality of terminals disposed on a flexible,sheet-like top layer. In use, the interposer is disposed on the front orcontact-bearing surface of the chip with the terminals facing upwardly,away from the chip. The terminals are then connected to the contacts ofthe chip. Most preferably, this connection is made by bondingprefabricated leads on the interposer to the chip contacts, using a toolengaged with the lead. The completed assembly is then connected to asubstrate, as by bonding the terminals of the chip carrier to thesubstrate. Because the leads and the dielectric layer of the chipcarrier are flexible, the terminals on the chip carrier can moverelative to the contacts on the chip without imposing significantstresses on the bonds between the leads and the chip, or on the bondsbetween the terminals and the substrate. Thus, the assembly cancompensate for thermal effects. Moreover, the assembly most preferablyincludes a compliant layer disposed between the terminals on the chipcarrier and the face of the chip itself as, for example, an elastomericlayer incorporated in the chip carrier and disposed between thedielectric layer of the chip carrier and the chip. Such a compliantstructure permits displacement of the individual terminals independentlytowards the chip. This permits effective engagement between thesubassembly and a test fixture. Thus, a test fixture incorporatingnumerous electrical contacts can be engaged with all of the terminals inthe subassembly despite minor variations in the height of the terminals.The subassembly can be tested before it is bonded to a substrate so asto provide a tested, known, good part to the substrate assemblyoperation. This in turn provides very substantial economic and qualityadvantages.

[0008] Commonly owned U.S. Pat. No. 5,455,390 describes a furtherimprovement. Components according to preferred embodiments of the '390patent use a flexible, dielectric top sheet having top and bottomsurfaces. A plurality of terminals is mounted on the top sheet. Asupport layer is disposed underneath the top sheet, the support layerhaving a bottom surface remote from the top sheet. A plurality ofelectrically conductive, elongated leads are connected to the terminalson the top sheet and extend generally side by side downwardly from theterminals through the support layer. Each lead has a lower end at thebottom surface of the support layer. The lower ends of the leads haveconductive bonding materials as, for example, eutectic bonding metals.The support layer surrounds and supports the leads.

[0009] Components of this type can be connected to microelectronicelements such as semiconductor chips or wafers by juxtaposing the bottomsurface of the support layer with the contact-bearing surface of thechip so as to bring the lower ends of the leads into engagement with thecontacts on the chip, and then subjecting the assembly to elevatedtemperature and pressure conditions. All of the lower ends of the leadsbond to the contacts on the chip substantially simultaneously. Thebonded leads connect the terminals of the top sheet with the contacts onthe chip. The support layer desirably is either formed from a relativelylow-modulus, compliant material, or else is removed and replaced afterthe lead bonding step with such a compliant material. In the finishedassembly, the terminals desirably are movable with respect to the chipto permit testing and to compensate for thermal effects. However, thecomponents and methods of the '390 patent provide further advantages,including the ability to make all of the bonds to the chip or othercomponent in a single lamination-like process step. The components andmethods of the '390 application are especially advantageous when usedwith chips or other microelectronic elements having contacts disposed inan area array.

[0010] Despite the positive results of the aforementioned commonly ownedinventions, still further improvements would be desirable.

SUMMARY OF THE INVENTION

[0011] The present invention contemplates a method of creating acompliant semiconductor chip package assembly and the semiconductor chippackage assembly created therefrom.

[0012] In a fabrication process according to one aspect of theinvention, a first dielectric protective layer is provided on a contactbearing surface of a semiconductor chip. The semiconductor chip has acentral region bounded by the chip contacts and a set of apertures. Theapertures in the dielectric protective layer are provided such that thechip contacts are exposed. This first dielectric protective layer mayactually be the silicon dioxide passivation layer of the semiconductorchip.

[0013] Second, a compliant layer, preferably consisting of silicone,flexibilized epoxy, a thermosetting polymer or polyimide is providedatop the first dielectric protective layer is provided within thecentral region. The compliant layer is formed such that it has asubstantially flat top surface and edges that gradually slope down tothe top surface of the first dielectric protective layer. The slopingedges of the compliant layer may be manufactured to have a firsttransition region near the top surface of the compliant layer and asecond transition region near the bottom surface of the compliant layersuch that both the first transition region and the second transitionregion have a radius of curvature.

[0014] Finally, bond ribbons are selectively formed atop both the firstdielectric protective layer and the compliant layer such that each bondribbon electrically connects each chip contact to a respective terminalposition on the compliant layer. The bond ribbons may be selectivelyformed using a variety of techniques, such as by electroplating or byelectroless plating followed by selective etching. The terminalpositions are the conductive elements that connect the finished assemblyto a separate substrate, e.g. a printed circuit board.

[0015] The method described above may further include the step ofproviding for a second dielectric protective layer atop the bond ribbonsand the compliant layer after the bond ribbon electroplating step isperformed. This optional second dielectric protective layer isfabricated with a set of apertures that expose the underlying terminalpositions on the compliant layer.

[0016] Additionally, the method described above may further include theoptional step of providing for an encapsulant layer above the bondribbons. If this optional step is performed, it is performed after thestep of selectively electroplating the bond ribbons. Like the firstdielectric layer, the encapsulant layer is fabricated with a set ofapertures so that the terminal positions are exposed. The encapsulantlayer material consists preferably of either a curable liquid, such assilicone, a flexibilized epoxy or a gel. This optional step may also beperformed just prior to the optional step of providing for a seconddielectric protective layer.

[0017] In another aspect of the invention, a method of making acompliant microelectronic assembly includes providing a microelectronicelement, such as a semiconductor chip, having a first surface and aplurality of contacts disposed on the first surface thereof and forminga compliant layer over the first surface of the microelectronic element.The compliant layer typically has a bottom surface facing toward thefirst surface of the microelectronic element, a top surface facingupwardly away from the microelectronic element and one or more edgesurfaces extending between the top and bottom surfaces. The edgesurfaces of the compliant layer are preferably sloping surfaces thatextend in both vertical and horizontal directions. At least some of thesloping edge surfaces preferably have first transition regions near thetop surface of the compliant layer and second transition regions nearthe bottom surface of the compliant layer, the first and secondtransition regions having respective radii of curvature.

[0018] In certain embodiments, before the compliant layer is formed, afirst dielectric protective layer, such as a layer including a silicondioxide passivation layer, may be provided on the first surface of themicroelectronic element. The first dielectric protective layer may havea plurality of apertures therein so that the contacts are accessibletherethrough. The compliant layer described above can then be providedover the dielectric protective layer.

[0019] Bond ribbons may then be selectively formed over the compliantlayer. The bond ribbons preferably extend over both the top surface ofthe compliant layer and one or more edge surfaces of the compliantlayer. The bond ribbons electrically connect the contacts to conductiveterminals overlying the top surface of the compliant layer. Before thebond ribbons are formed, a barrier metal layer may be deposited over thecontacts so as to prevent undesired chemical reactions between thecontacts and the bond ribbons. In one embodiment, the bond ribbons areformed by selectively electroplating the bond ribbons atop the firstdielectric protective layer and the compliant layer. After the bondribbons have been formed, a dielectric cover layer may be formed overthe compliant layer and the bond ribbons. The dielectric cover layer mayhave a plurality of apertures therein so that the terminals areaccessible therethrough. In other embodiments, an encapsulant layer maybe provided atop the exposed surfaces of the bond ribbons. Theencapsulant layer is generally a material selected from the groupconsisting of silicone, flexibilized epoxy, thermoplastic and gel. Next,a second dielectric protective layer or cover layer may be provided overthe encapsulant layer. The second dielectric protective layer alsopreferably has a plurality of apertures therein so that the terminalsare accessible therethrough.

[0020] The compliant layer may include one or more apertures therein sothat the contacts are accessible through the apertures. The one or moreapertures may include one or more groups of apertures encompassing aplurality of the contacts. The edge surfaces of the compliant layer mayinclude one or more aperture edge surfaces bounding the apertures, withat least some of the bond ribbons being formed over the aperture edgesurfaces. The compliant layer may be formed by engaging themicroelectronic element with a mold so that one or more projections onthe mold contact the first surface of the microelectronic element. Aflowable composition may be introduced around the projections and theflowable composition set to provide a compliant layer. Themicroelectronic layer may then be separated from the mold. The one ormore apertures are typically formed in the space occupied by theprojections.

[0021] In certain embodiments, the contacts on the microelectronicelement are disposed in an area array, and the one or more apertures inthe compliant layer include a plurality of apertures disposed in anarray corresponding to the array of contacts so that each contact isencompassed in a respective aperture. In other embodiments, the contactson the microelectronic element may be disposed in a first region of thefirst surface, with the compliant layer overlying a second region of thefirst surface, and one or more edge surfaces including one or moreborder edge surfaces extending along one or more borders between thefirst and second regions. In still other embodiments, the contacts onthe microelectronic element are disposed in a central region of thefirst surface and the compliant layer overlies a peripheral region ofthe first surface.

[0022] In another embodiment, a method of making a compliantmicroelectronic package includes providing a supporting element havingan upwardly-facing top surface and juxtaposing a microelectronic elementincluding a first surface having a plurality of contacts thereon withthe supporting element so that the first surface of the microelectronicelement is disposed alongside the top surface of the supporting element.The first surface of the microelectronic element and the top surface ofthe supporting element may be substantially coplanar after thejuxtaposing step.

[0023] A compliant layer may then be provided over the top surface ofthe supporting element, the compliant layer having a top surface remotefrom the top surface of the supporting element, a bottom surface and anedge surface extending between the top surface and the bottom surface.In certain embodiments, a portion of the compliant layer extends overthe first surface of the microelectronic element, with one or more edgesurfaces of the compliant layer overlying the first surface of themicroelectronic element. Bond ribbons may then be selectively formedatop the compliant layer, the bond ribbons electrically interconnectingthe contacts of the microelectronic element with conductive terminalsoverlying the top surface of the compliant layer.

[0024] The supporting structure described above may have a centralaperture therein so that the microelectronic element may be placed inthe central aperture after being juxtaposed with the supporting element.After the juxtaposing step, the first surface of the microelectronicelement and the top surface of the supporting structure are preferablysubstantially coplanar.

[0025] In certain embodiments, the compliant chip assembly may include aground plane electrically interconnected with at least one of the bondribbons. The ground plane may include a plurality of apertures thereinso that the terminals are accessible through the apertures.

[0026] The methods described above can be applied simultaneously to amultiplicity of undiced semiconductor chips on a wafer to form acorresponding multiplicity of compliant semiconductor chip packages.After the bond ribbons have been formed on the packages, individualpackages may be severed or diced from the wafer to provide separate anddistinct chip packages. The methods may also be applied to amultiplicity of adjacent semiconductor chips arranged in an array toform a corresponding multiplicity of compliant semiconductor chippackages, whereby the packages are diced after the bond ribbons havebeen formed.

[0027] A further aspect of the present invention includes the structureof a unique compliant semiconductor chip package having fan-in typeleads. The compliant semiconductor chip package is comprised of (1) asemiconductor chip having a plurality of peripheral bonding pads on aface surface thereof and a central region bound by the peripheralbonding pads; (2) a first dielectric protective layer having a firstsurface, a second surface and apertures, wherein the first surface ofthe first dielectric layer is joined to the face surface of thesemiconductor chip and the peripheral bonding pads are exposed throughthe apertures; (3) a compliant layer having a top surface and a bottomsurface, wherein the bottom surface of the compliant layer is joined tothe second surface of the first dielectric layer within the centralregion of the semiconductor chip package; and (4) a plurality ofelectrically conductive bond ribbons, each bond ribbon having a firstend that electrically couples to a respective peripheral bonding pad ofthe semiconductor chip and a second end that joins to the top surface ofthe compliant layer to form a package terminal.

[0028] The package terminals of the completed package are configured inan array that has an area smaller than the area bound by the peripheralbonding pads on the face of the semiconductor chip. In other words, thepackage has fan-in leads that permit minimization of the overall packagesize.

[0029] For increased reliability, the compliant layer has slopedperipheral edges so that the overlying bond ribbons are curved ratherthan kinked.

[0030] The compliant semiconductor chip package may also have acompliant layer characterized by an array of bumped protrusions. Thebumped protrusions support the overlying conductive terminal positionends of the bond ribbons and function as conductive balls that join to asubstrate thus forming a ball grid array type interconnection. Alternateto the bumped protrusions, the compliant layer may have an array ofconcavities that are useful for placement of solder balls into eachconcavity. This arrangement is also useful for a ball grid array typeinterconnect.

[0031] The foregoing and other objects and advantages of the presentinvention will be better understood from the following DetailedDescription of a Preferred Embodiment, taken together with the attachedfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1A is a cross-sectional view of a semiconductor chip assemblyat the beginning of a fabrication process.

[0033]FIG. 1B is a cross-sectional view of the semiconductor chipassembly after a first step of the fabrication process, showing adeposited or laminated dielectric passivation layer.

[0034]FIG. 1C is a cross-sectional view of the semiconductor chipassembly after a second step of the fabrication process, showing adeposited or laminated compliant layer within the central region of thesemiconductor chip contact-bearing surface.

[0035]FIG. 1D is a cross-sectional view of the semiconductor chipassembly after a third step of the fabrication process, showing aconductive seed layer that has been sputtered over the assembly.

[0036]FIG. 1E is a cross-sectional view of the semiconductor chipassembly after a fourth step of the fabrication process, illustratinghow after a photolithographic step conductive bond ribbons can be formedover the assembly.

[0037]FIG. 1F is a cross-sectional view of the semiconductor chipassembly after a fifth step of the fabrication process, showing how theassembly is coated with a second dielectric protective layer.

[0038]FIG. 2 is a perspective view of the semiconductor chip assemblyafter the bond ribbons have been formed over the compliant layer butbefore the second dielectric protective layer is coated.

[0039]FIG. 3 is a plan view of a wafer having a multiplicity ofsemiconductor chips, illustrating how said multiplicity of semiconductorchips can be simultaneously packaged using the semiconductor chipassembly process depicted in FIGS. 1A-1F.

[0040]FIG. 4 is a cross-sectional view of an alternate embodiment of thepresent invention, illustrating the use of a low modulus encapsulantmaterial to provide further support and stress relief to the bondribbons.

[0041]FIG. 5A is a cross-sectional view of an alternate embodiment ofthe present invention, illustrating the formation of bumped protrusionsin the compliant layer that raise the overlying terminals such that theterminals form an array over the top surface of the compliant layer.

[0042]FIG. 5B is a perspective view of the embodiment shown in FIG. 5A.

[0043]FIG. 6A is a cross-sectional view of an alternate embodiment ofthe present invention, illustrating the formation of concave areas inthe compliant layer such that the overlying terminals have cup-likedepressions useful for accurate placement of solder balls.

[0044]FIG. 6B is a perspective view of the embodiment shown in FIG. 6A.

[0045]FIG. 7A is a cross-sectional view of a first step of asemiconductor chip assembly process according to another embodiment ofthe present invention.

[0046]FIG. 7B is a cross-sectional view of the assembly shown in FIG.7A, showing a mold for forming a compliant layer on top of the assembly.

[0047]FIG. 7C is a cross-sectional view of the assembly shown in FIG. 7Bafter conductive bond ribbons have been formed atop the compliant layer.

[0048]FIG. 7D shows the assembly of FIG. 7C after the top of theassembly has been coated with an additional dielectric protective layer.

[0049]FIG. 8A is a perspective view of the semiconductor chip assemblyshown in FIG. 7C, before the additional dielectric protective layer hasbeen provided over the bond ribbons.

[0050]FIG. 8B is a close-up, fragmentary, cross-sectional view of theassembly shown in FIG. 7D.

[0051]FIG. 9 is a cross-sectional view of another embodiment of thepresent invention, which includes a semiconductor chip having aplurality of contacts in a central region thereof.

[0052]FIG. 10 is a perspective view of the assembly shown in FIG. 9.

[0053]FIG. 11 is a top view of a semiconductor chip having a pluralityof non-uniform, staggered chip contacts in a peripheral region of asemiconductor chip, in accordance with another embodiment of the presentinvention.

[0054]FIG. 12 is a fragmentary top view of the chip shown in FIG. 11after a compliant layer and bond ribbons have been formed atop the chip.

[0055]FIG. 13 is a cross-sectional view of a compliant chip assemblyhaving a supporting element with a central opening and a semiconductorchip provided in the central opening of the supporting element inaccordance with yet another embodiment of the present invention.

[0056]FIG. 14 is a cross-sectional view of a compliant chip assemblyincluding a flexible dielectric sheet in accordance with still anotherembodiment of the present invention.

[0057]FIG. 15 is a cross-sectional view of a compliant chip assemblyincluding a ground plane in accordance with a further embodiment of thepresent invention.

[0058]FIG. 16 is a fragmentary top view of the compliant chip assemblyshown in FIG. 15.

[0059]FIG. 17 is a cross-sectional view of a compliant chip assemblyincluding a ground plane in accordance with still further embodiments ofthe present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0060] FIGS. 1A-F illustrate a side view of the process of creating thecompliant chip package of the present invention on the face surface of asingle die, on the face surfaces of multiple die arranged in a coplanararray or on the face surface of an undiced silicon wafer which may besubsequently diced into individual packaged chips or multi-chip modules.

[0061]FIG. 1A shows a single semiconductor chip 100 with a contactbearing face surface 120. The contacts 110 on the face surface 120 aretypically aligned in a peripheral region 112 and further define acentral region 115 therein. In FIG. 1B, a dielectric passivation layeris deposited or adhered onto the face surface 120 of the chip 100. Thepassivation layer may simply be the SiO₂ passivation layer (not shown)commonly found on the contact bearing surface of semiconductor chips, ora separate dielectric passivation layer 130 may be used, such as anepoxy resin, a polyimide resin, photo-imagable dielectric, etc. If theseparate passivation layer 130 is used, the passivation layer 130 may bespun onto and built up to a planar sheet-like form on the face surface120 or a dielectric sheet may be laminated to the face surface 120 usingany of a number of electronic grade adhesives commonly known and used bythose skilled in the art. The passivation layer 130 covers the facesurface 120 of the chip 100 while leaving the chip contacts 110 exposedso that a bond ribbon may be plated thereon in a later step, asdescribed below. Typically, this will be done by depositing or adheringthe passivation layer 130 in a continuous sheet on the face surface 120of the chip 100. A registering system, such as an automatic visionsystem, is used to locate the contacts 110. If a photo-imagabledielectric is used, the passivation layer 130 may be exposed anddeveloped without exposing the area above the contacts 110, thatunexposed area may then be removed. Another removal process that can beused is to use a pulse of directed energy, such as an excimer laser, toselectively remove the passivation layer 130 above the contacts 110.Alternately, a continuous dielectric sheet already having set contactholes may be registered and laminated to the chip 100.

[0062] In the next step, as illustrated in FIG. 1C, a compliant layer140 is deposited or laminated onto the exposed surface of thepassivation layer 130. The compliant layer 140 may be stenciled,screened or transfer molded onto the passivation layer 130 using acurable liquid which, when cured, adheres to the passivation layer 130.Alternately, the compliant layer 140 may be adhered to the exposedsurface of the passivation layer 130 in the form of cured compliant padsusing the aforementioned electronic grade adhesives. The compliant layer140 has a substantially flat top surface 147, which further typicallyhas a gradual, sloping transition 145 between the face surface 120 ofthe chip 100 and the top surface 147. This transition 145 may follow aline of curvature from the passivation layer 130 to a substantially flattop surface 147 or may simply be canted at an angle such that thetransition 145 is not too vertically oriented in relation to thepassivation layer 130 and the top surface 147. The compliant layer 140itself may be formed from a wide variety of materials; however,preferably, a low modulus of elasticity material is used as thecompliant layer 140. Compliant interposers typically are fabricated frompolymeric and other materials such as silicones, flexibilized epoxy,polyimides and other thermosetting polymers, fluoropolymers andthermoplastic polymers. Also, the interposer may be a compositeincorporating plural materials. The interposer may consist of, orincorporate, a foam or mesh layer. The flexibility of the interposerdepends on the thickness and configuration of the interposer, as well ason the properties of the materials used therein. Thus, a flexibleinterposer, capable of buckling or wrinkling to accommodate relativemovement, can be fabricated from high elastic modulus materials,normally considered as “rigid” provided that these materials are presentin thin layers. Relatively soft materials and foams can be used ingreater thicknesses and still provide a highly flexible interposer.Moreover, such soft materials and foams provide a highly compliantinterposer, i.e., an interposer that is readily compressible in thedirections perpendicular its surfaces and which therefore permitsmovement of the terminals in these directions.

[0063] A plating seed layer 150 is then deposited atop theaforementioned assembly, as shown in FIG. 1D, typically using asputtering operation. Typical plating seed layer materials includepalladium (for electroless plating), titanium, tungsten, nickel, andchromium; however, primarily copper seed layers are used. FIG. 1E showsthe next step in which photoresist 160 is applied to the exposed topsurfaces of the assembly and then exposed and developed such that bondribbons 170 may be plated within defined areas to form conductive pathselectrically connecting the chip contacts 110 near a first end region ofthe ribbons 170 to terminals 175 comprising the second end region of theribbons 170. This is perhaps more easily seen in the perspective viewshown in FIG. 2. As shown, the ribbons 170 are plated directly onto thecontacts 110 and extend in a “fan-in” arrangement from the peripheralregion 112 to the central region 115 of the face surface 120 of the chip100 atop the compliant layer 140. Possible bond ribbon materials includecopper, gold, nickel, and alloys, combinations and composites thereof,among others. Since the bond ribbons 170 are plated directly onto thechip contact/compliant layer themselves, there is no need to develop aprocess for bonding the ribbons 170 to the contacts, as is necessarywith most other approaches such as TAB, beam lead or wirebonding. Thisprovides a significant cost savings because specializedthermocompression or ultrasonic bonders and their bonding tools need notbe purchased or maintained. It is important, however, that the materialselected for the bond ribbon 170 be compatible with the chip contact 110material, which is typically aluminum. Otherwise, a phenomenon calledKirkendahl Voiding (voids created at the boundary of two metals havingdifferent interdiffusion coefficients) may cause voiding along theboundary of the two metals (ribbon/contact) leading to intermetallicdegradation and embrittlement of the bond ribbon 170 itself making thelead/bond susceptible to failure during thermal cycling. Alternately,one or more barrier metals may be plated atop the chip contacts 110prior to the bond ribbon plating step to thereby ensure thecompatibility of materials.

[0064] As shown in FIG. 1F, preferably, a dielectric layer 180 isdeposited or laminated over the top of the assembly so that only theterminals 175 are exposed. The dielectric layer may be comprised of ascreened, exposed and developed or laminated sheet photo resist materialor may be comprised of paralyne, epoxy resin, polyimide resin,fluoropolymer, etc. which is deposited or laminated on to the assembly,as described above in relation to the passivation layer 130. Theterminals 175 may then be electrically connected to a circuitizedsubstrate, such as a printed wiring board.

[0065] Typically, a solder ball or a solid-core solder ball will be usedto create this electrical connection. The dielectric layer 180 is thusused as a solder mask to ensure that the solder does not electricallyshort between adjacent bond ribbons 170. Oxide layers and other surfacecontaminates typically build up on the surface of many types of metal(copper, nickel, etc.). Although not shown in FIG. 1F, the terminals 175are typically flash plated with a thin layer of gold (approximately 0.25to 0.5 microns) to inhibit the formation of these oxide layers. The goldlayer is kept very thin so that it does not appreciably affect theaforementioned solder joint by dissolving into the solder to an amountwhich would embrittle the resulting solder joint between the terminaland a circuitized substrate.

[0066] The configuration of the above described chip package allows thepackage to mechanically decouple the chip 100 from an attachedcircuitized substrate (not shown). Typically, solder connections betweenthe chip and the circuitized substrate are woefully inadequate tocompensate for the thermal mismatch problem during temperature cyclingof the chip. The combination of the compliant layer 140 and the flexiblebond ribbons plated thereon allow the package to compensate for much ofthe TCE mismatch problem by giving limited movement of the terminals inthe X, Y and Z directions with respect to the chip contacts 110 therebyminimizing the stress placed on the solder connections themselves,without imposing substantial forces on the bond between the ribbons 170and the chip contacts 110. Further, because the compliant layer 140 iscompressible, it also has the effect of compensating for any terminals175 which are not perfectly planar with respect to its adjacentterminals when the terminals 175 are abutted against and coupled to thecircuitized substrate. However, the top surface 147 of the compliantlayer 140 should be made as flat and planar as possible so that theterminals 175 all lie in or near the same plane in order to minimize theamount of pressure needed to be placed on the bottom surface 125 of thechip 100 to ensure that all of the terminals/solder balls areelectrically connected to a circuitized substrate.

[0067] As illustrated in FIG. 3, the chip package described above inrelation to FIGS. 1 and 2 may also be provided in the form of amultiplicity of packages on a wafer incorporating a plurality ofindividual, undiced chips, all of the same design or of differingdesigns. As shown, an array of individual passivation layers 230 may bedeposited or laminated onto the face surface 220 of the wafer 200leaving the chip contacts 210 of the various individual chips exposed,as described above. This arrangement is shown to better define theindividual chips within the wafer. Preferably, however, a singlepassivation layer 230 is deposited or laminated onto the face surface220 leaving the contacts 210 exposed. Individual compliant layers 240,as described above, are deposited or laminated onto the central regionsof each of the individual chips within the wafer 200. The steps found inFIG. 1A-F are then performed, as described above, to create a pluralityof connected individually packaged chips on the face surface 220 of thewafer 200. Each packaged chip having bond ribbons 270 which areconnected at one end to contacts 210 and extending in to a centralregion of the respective chip in a fan-in fashion atop a respectivecompliant layer 240 and ending with a terminal 275 on the top surface247 of the compliant layer 240. After the individual packages arecompleted, the individual chips may be separated from the wafer 200 andfrom one another, as by cutting the wafer 200 using conventional wafersevering or “dicing” equipment commonly utilized to sever wafers intoindividual chips. This procedure yields a plurality of packaged chipsubassemblies, each of which may be secured to an individual circuitizedsubstrate. Alternately, the chips may be separated from the wafer 200 inmulti-chip arrangements of multiples of the same or differentoperational chips. The wafer level embodiment shown in FIG. 3 could besimulated using a panel of individual chips spaced apart from oneanother in a processing boat. The face surfaces of the individual chipswould be coplanar with respect to one another to simulate the facesurface 220 of the wafer 200. The chips above described steps would beperformed and the chips would be separated if desired.

[0068] In the alternate embodiment shown in FIG. 4, a low modulusencapsulant material 290 may be deposited around the exposed surfaces ofthe bond ribbons 170′ leads prior to the step shown in FIG. 1F ofdepositing or laminating the assembly with the dielectric layer 180′.The encapsulant material 290 may have properties similar to those ofrubber, gum or gel. Typical encapsulation materials include curableliquid or cured pads comprised of silicone, flexibilized epoxy, gels,thermoplastics, etc. If the encapsulant 290 is applied as a curableliquid, a fixture may be made such that the liquid flows around the bondribbons 170′ but does not flow on top of the terminals 175′ to ensurethat solder balls may be subsequently electrically connected to theterminals 175′, as described above. Alternately, a machine such as aCamalot 1818 manufactured by Camalot Systems, Inc. of Havermill, Mass.may be used to flow the liquid encapsulant into the desired areas. Afterthe liquid is deposited, it may be cured by any number of ways dependingon the encapsulant material 290 used, e.g. heat, infrared energy, etc.The encapsulant 290 gives each of the bond ribbons 170′ more support andfurther spreads some of the stress away from the ribbons 170′ thusallowing a larger TCE mismatch between the chip and a circuitizedsubstrate, as described above. After curing of the encapsulant 290, thedielectric layer 180′ may be deposited or laminated thereto.

[0069] In another alternate embodiment, a conductive material such asberyllium copper, or a super plastic or shape memory alloy (such asNitinol), is sputtered or otherwise deposited across the entire exposedsurface of the chip/passivation layer/compliant layer (100/130/140)combination, shown in FIG. 1C. The conductive material may then beetched using industry standard photolithographic techniques resulting ina multiplicity of bond ribbons positioned and configured much like thebond ribbons 170 shown in FIG. 1E and FIG. 2. In this embodiment, asdescribed above, a barrier metal, such as a flash plated layer of gold,may first be plated to the chip contacts to ensure compatibility of theelectrical connection between the chip contact and the bond ribbon.Likewise, a flash plated layer of gold may be plated atop the exposedsurface of the terminal. Also, the entire exposed surface of the bondribbon could be plated with a thin layer of gold to increase the overallconductivity of such super plastic leads. A dielectric layer is nextdeposited or laminated as shown in FIG. 1F.

[0070]FIG. 5A shows a side view and FIG. 5B a perspective view ofanother embodiment, according to the present invention. In thisembodiment, the compliant layer 140′ has protrusions 300 on its topsurface 147′. These protrusions 300 may be integral with the compliantlayer 140′ or may be deposited or laminated onto the top surface 147′subsequent to the formation of the compliant layer 140′. The protrusions300 may be formed of compliant, elastomeric material, such as thematerial comprising the compliant layer 140′, or may be comprised of asemi-rigid or rigid material. The bond ribbon terminals 175′ are platedon top of the protrusions 300 thereby providing raised surfaces that maybe connected to a circuitized substrate. This technique allows forconnection to such a substrate using less solder and without the need toaccurately position solid-core solder balls.

[0071]FIG. 6A shows a side view and FIG. 6B a perspective view ofanother embodiment, according to the present invention. In thisembodiment, concave areas 310 are created in the compliant layer 140″.These concave areas 310 may be create in the formation of the compliantlayer 140″ or may be created subsequent to the formation of thecompliant layer 140″. The bond ribbon terminals 175″ are plated withinthe concave areas 310 creating conductive “cup-like” areas on the topsurface 147″ of the compliant layer 140″. Solder or solid-core solderballs are then placed within these areas 310 and reflowed to attach thepackage to a circuitized substrate, as described earlier. This techniqueallows for the accurate placement of solder or solid-core solder ballsby allowing them to be deposited and retained within the cup-like areas.

[0072] FIGS. 7A-7D illustrate a side view of a method of making acompliant microelectronic package including a semiconductor chip havingan area array of contacts on a first surface thereof. The package ispreferably assembled by using the method steps described above.

[0073]FIG. 7A shows a single semiconductor chip 400 having a firstsurface 420 including a plurality of contacts 410 provided in an areaarray over the first surface 400. A dielectric passivation layer 430 isdeposited over the first surface 420 of the chip 400 and preferablycovers the first surface 420 of the chip 400 while leaving the chipcontacts 410 exposed so that a bond ribbon (not shown) may be platedthereon, as will be described in more detail below.

[0074] Next, as illustrated in FIG. 7B, the chip 400, including thepassivation layer 430, is placed in a mold 488 so that a compliant layermay be formed atop the passivation layer 430. The compliant layer 440 ispreferably molded onto the passivation layer 430 using a curable liquidwhich, when cured, adheres to the passivation layer 430. In onepreferred embodiment, the mold 488 has downwardly extending projections489 which are shaped to completely cover the chip contacts 410 when themold 488 is in a closed position. The mold 488 includes open spaces 493between the projections 489. In order to form the compliant layer 440,the chip 400 is placed in a frame 491 and the mold is closed on top ofthe chip 400 so that the projections 489 completely cover the contacts410. Next, a curable liquid 440 is introduced into the mold and fillsthe open spaces 493 between the projections 489. The curable liquid isthen cured while the mold remains in the closed position so as to formthe compliant layer 440 having a substantially flat top surface 447including a plurality of openings 495 aligned with the contacts 410. Theheight of projections 489 is exaggerated in FIG. 7B for clarity ofillustration. In practice, projections 489 typically are about 75-200microns high, and hence compliant layer 440 typically is about 74-200microns thick. In each opening 495 has a gradual sloping edge 497 ortransition between the first surface 420 of the chip 400 and the topsurface 447 of the compliant layer 440. This sloping edge 497 willpreferably follow a line of curvature from the passivation layer 430 tothe substantially flat top surface 447, or may simply be canted at anangle such that the sloping edge 497 is not too vertically oriented inrelation to the passivation layer 430 and the top surface 447 of thecompliant layer. For example, sloping edge 497 typically is disposed atan angle of about 20-70° to the plane of the chip front surface, andmore typically about 40-60°. Also, the sloping surface typically iscurved to define a radius at the juncture of sloping surface 497 and topsurface 447. A further radius or fillet can be provided at the junctionof the sloping surface and the front surface of the chip.

[0075] In the next step, illustrated in FIG. 7C, bond ribbons 470 areselectively formed within defined areas to create conductive pathselectrically connecting the chip contacts 410 near a first end of thebond ribbons 470 to conductive terminals 475 at a second end of the bondribbons. In certain embodiments, the bond ribbons 470 may be formedusing selective electroplating or other selective deposition techniques.In other embodiments, the selection forming step used to make bondribbons 470 may include one or more non-selective deposition techniquessuch as electroless plating or sputtering of a conductive layer over theassembly, with or without an additional non-selective electroplatingstep, followed by selectively etching of the conductive layer to provideelectrically isolated bond ribbons. FIG. 8A shows a perspective view ofthe bond ribbons after they have been selectively formed over thecompliant layer. In alternative embodiments, one or more barrier metallayers (not shown) may be plated atop the chip contacts 410 prior toforming the bond ribbons 470 so as to insure the compatibility ofmaterials.

[0076] Referring to FIG. 7D, a dielectric layer 480 is then deposited orlaminated over the top of the assembly so that only the conductiveterminals 475 are accessible at the top of the assembly. The terminals475 may then be electrically interconnected with an external circuitelement, such as a printed circuit board. Typically, a solder ball orsolid core solder ball will be used to create this electricalconnection. Thus, the dielectric layer 480 serves as a solder mask,thereby insuring that the solder does not electrically short betweenadjacent bond ribbons 470.

[0077]FIG. 8B shows a close-up, fragmentary, cross-sectional view ofFIG. 7D. The assembly includes the compliant layer 440 having aplurality of apertures 495 therein so the contacts 410 are accessiblethrough the apertures 495. Each aperture 495 in the compliant layer 440preferably includes at least one sloping edge side wall 497 thatprovides a gradual sloping transition between the first surface 420 ofthe chip 400 and the top surface 447 of the compliant layer 440. Thetransition preferably follows a line of curvature from the passivationlayer 430 to the top surface 447 or may simply be canted at an angle sothat the transition from the first surface 420 of the chip 400 to thetop surface 447 of the compliant layer 440 is not too verticallyoriented in relation to the passivation layer 430. The top surface 447of the passivation layer is preferably substantially flat, however, incertain embodiments the top surface 447 may be slightly rounded. Asstated above, the low points in the compliant layer may next be filledwith compliant material to encase the leads and/or cover sheets ofmaterial.

[0078] As illustrated in FIG. 9, a compliant chip package in accordancewith another preferred embodiment of the present invention includes asingle semiconductor chip 500 having a first surface 520 with a first orcentral region 515 and a second or peripheral region 517 surrounding thecentral region 515. The chip 500 includes a plurality of contacts 510disposed in the central region 515 thereof. A passivation layer 530 ispreferably deposited over the first surface 520 of the chip 500. Thepassivation layer 530 includes apertures aligned with the contacts 510so that the chip contacts 510 are accessible through the passivationlayer 530. A compliant layer 540 is then formed over the passivationlayer, the compliant layer having openings 595 in alignment with thechip contacts 510 so that the contacts are accessible through thecompliant layer openings 595. The steps described above are thenperformed to create a plurality of bond ribbons 570 which are connectedat one end to the chip contacts 510 and at a second end to conductiveterminals 575 accessible at the substantially flat surface 547 of thecompliant layer 540. The final assembly provides a compliant chippackage having a plurality of contacts 510 in the central region 515thereof and bond ribbons 570 extending outwardly from the contacts 510to conductive terminals 575 overlying the peripheral region 517 of thechip 500. The centrally located low point in the compliant layer can befilled in with compliant material to encapsulate the leads.

[0079]FIG. 10 shows a perspective view of the package illustrated inFIG. 9. As shown in FIG. 10, the plurality of contacts 510 is located inthe central region 515 of the chip 500. Compliant layer 540 defines twosloping edges 572 at the border of the first or central region of thechip surface and the second or peripheral region. Bond ribbons 570 havefirst ends electrically connected to the contacts 510 and second endsextending to conductive terminals 575 provided at the top surface 547 ofthe compliant layer 540. The specific embodiment shown in FIG. 10includes a compliant layer 540 having a first section on the left sideof the chip 500 and a second section on the right side of the chip 500,however, other preferred embodiments may include compliant layers havingmore than two distinct portions.

[0080] In still another embodiment, illustrated in FIG. 11, a compliantchip package includes a semiconductor chip 600 having a first surfacewith a central region 615 and a peripheral region 612 surrounding thecentral region 615. The peripheral region 612 includes a plurality ofcontacts 610 which are arranged in a staggered or non-uniformconfiguration. In other words, the peripheral region 612 includescontacts 610 which are positioned at non-uniform distances from an edge617 of the chip 600. In other embodiments, the chip may include contactsclumped together in groups and/or disposed in a non-uniform patternthroughout the entire first surface of the chip.

[0081] The method steps described above are then utilized to provide afinal compliant chip package, as shown in FIG. 12, whereby the contacts610 are positioned at varying distances from the edge 617 of the chip600. FIG. 12 shows four different contacts, designated 610A-610D,located in the peripheral region 612 of the chip 600. The contacts arestaggered with respect to one another so that contacts 610B and 610D arecloser to the edge 617 of the chip than contacts 610A and 610C. Thecontacts 610 are electrically connected to terminals 675 by bond ribbons670. The actual length of bond ribbons 670 may vary based upon theposition of the contact 610 and the desired position of the terminal675. For example, although contacts 610A and 610C are positioned at auniform distance from the edge 617 of the chip 600, bond ribbon 670C islonger than bond ribbon 670A. As a result, the terminal 675C connectedto bond ribbon 670C may be positioned at a more central location thanterminal 675A. The ability to modify the length of the bond ribbons 670allows the terminals 675 to be positioned at an infinite number ofdifferent locations over the top surface 647 of the compliant layer 640so that the chip package can be reliably interconnected with an externalcircuit element, regardless of the location of contact pads on theexternal circuit element.

[0082] In a further embodiment, illustrated in FIG. 13, the compliantchip package includes a supporting element 792 adjacent a semiconductorchip 700, with conductive terminals 775 formed over a top surface 794 ofthe supporting element 792. The supporting element 792 may include a baror alternatively a ring having an opening 795 in the center thereof. Inthe latter embodiment, the semiconductor chip 700 is provided within theopening 795 so that a first contact bearing surface 720 of the chip 700is substantially parallel with the top surface 794 of the supportingelement 792. The first surface 720 of the semiconductor chip 700preferably includes a passivation layer 730 having openings therein sothat the contacts 710 are accessible through the openings. A compliantlayer 740 having a substantially flat top surface 747 and a bottomsurface and sloping edges 797 therebetween is then formed atop the topsurface 794 of the supporting element 792 and a portion of thepassivation layer 730. The compliant layer 740 preferably fills gaps 755between the peripheral edges of the chip 700 and the support element792. In addition, the compliant layer 740 preferably has ameniscus-shaped top surface so that the transition from the passivationlayer 730 to the compliant layer 740 is smooth. This smooth transitionwill increase the reliability of any bond ribbons formed atop thecompliant layer because the bond ribbons will be gently curved ratherthan kinked. Next, bond ribbons 770 are formed using the techniquesdescribed above, and a dielectric layer 780 is formed over the bondribbons 770 so that only conductive terminals 775 are accessible at thetop of the assembly. In certain embodiments the supporting element 792may include a heat sink and the compliant layer may be formed on the topsurface of flanges extending laterally from central opening in the heatsink.

[0083] In still another embodiment, illustrated in FIG. 14, thecompliant chip package includes a flexible dielectric sheet 865, such asa polyimide sheet, secured over the top of the compliant layer 840. Thepackage includes a semiconductor chip 800 having a first surface 820with contacts 810. A dielectric passivation layer 830, includingopenings in substantial alignment with the contacts 810, is then formedover the first surface 820 of the chip 800. After the compliant layer840 has been formed, the flexible dielectric sheet 865 is provided overthe top surface 847 of the compliant layer 840. The flexible dielectricsheet 865 generally improves the structural integrity of the package andprotects the compliant layer 840 from external contaminants. Bondribbons 870 are then formed atop the passivation layer 830, thecompliant layer 840 and the flexible dielectric sheet 865. The bondsribbons 870 have first ends which are connected to chip contacts 810 andsecond ends which provide conductive terminals 875 over the flexibledielectric sheet 865. In certain embodiments the dielectric sheet 865 isprovided as a separate sheet which is laminated or secured over the topsurface 847 of the compliant layer 840. In these embodiments, theconductive terminals 875 may be pre-formed on the dielectric sheet 865,with the bond ribbon forming step electrically interconnecting thecontacts 810 and the pre-formed conductive terminals 875. In stillfurther embodiments, the flexible dielectric sheet 865 may be spun ontothe top surface 847 of the compliant layer 840. As such, the edges ofthe spun-on dielectric sheet have radii of curvature which substantiallymatch the radii of curvature of the edges of the compliant layer. Thematched edges provide a smooth transition from the dielectric sheet 865to the compliant layer 840, thereby providing a more uniform surface forforming the bond ribbons 870. A second dielectric protective layer 880may then be formed over the bond ribbons 870 to further protect the bondribbons and electrically isolate the bond ribbons from one another.

[0084] In another embodiment, illustrated in FIGS. 15 and 16, acompliant chip package includes a ground plane 981. As shown in FIG. 15,a semiconductor chip 900 is provided within a central opening 995 of asupporting element 992 so that a first contact bearing surface 920 ofthe chip 900 is substantially parallel with a top surface 994 of thesupporting element 992. A first compliant layer 940 having asubstantially flat top surface 947 and a bottom surface and slopingedges 997 therebetween is then formed atop the top surface 994 of thesupporting element 992. The compliant layer 940 preferably fills gaps955 between the peripheral edges of the chip 900 and the support element992. The sloping edges 997 preferably provide a smooth transitionbetween the top surface 947 of the compliant layer 940 and the chip 900.Bond ribbons are then formed over the compliant layer 940 using thetechniques described above. The sloping edges 997 of the compliant layer940 will increase the reliability of the bond ribbons 970 because thebond ribbons will be gently curved rather than kinked. Next, a secondcompliant layer 941 is formed atop the bond ribbons 970 so as toencapsulate the bond ribbons 970. A dielectric layer 980 is then formedover the second compliant layer 941 and the bond ribbons 970 so thatonly conductive terminals 975 are accessible at the top of the assembly.The ground plane 981 is then provided atop the dielectric layer 980.Referring to FIG. 16, the ground plane preferably includes a highlyconductive material, such as copper, having a plurality of openingstherein. The openings are preferably formed using photolithographic andetching techniques. The openings are sized to fit over the terminals 975and a relatively small portion of the bond ribbon 970 extending awayfrom each terminal 975. The ground plane 981 is assembled to thedielectric layer by aligning the openings 983 therein with the terminals975 and abutting the ground plane 981 against the top of the dielectriclayer 980. The package is then subjected to a curing process so as tocure the compliant layers 940 and 941 and the dielectric layer 980.

[0085]FIG. 17 shows yet another embodiment of a compliant chip packagewhich is similar to that shown in FIG. 15, however, the FIG. 17embodiment lacks the top dielectric cover layer shown in FIG. 15.Referring to FIG. 17, after the second compliant layer 1041 is formedatop the bond ribbons 1070, a ground plane 1081, similar to that shownin FIG. 16 is provided over the second compliant layer 1041. Duringassembly, the ground plane 1081 may be compressed against the secondcompliant layer 1041 so that the ground plane 1081 is slightly sunk intothe second compliant layer 1041. The ground plane preferably includesopenings 1083 which are in alignment with terminals 1075 so that theterminals 1075 are accessible through the openings 1083. The package isthen subjected to a curing process so as to cure the compliant layers1040 and 1041. The ground plane 1081 is preferably electricallyconnected to at least one of the bond ribbons 1070.

[0086] These and other variations and combinations of the featuresdescribed above may be utilized without departing from the presentinvention as defined by the claims. For example, the low modulusencapsulant material shown in FIG. 4 may be used to assemble any of thecompliant chip packages shown in FIGS. 7A-14 to provide additionalstress relief for the bond ribbons. In addition, the assembly shown inFIG. 13 may be modified so as to provide conductive terminals over thecentral region of the chip, thereby providing a “fan-in/fan-out”compliant chip package. Moreover, all of the chip package assembliesdisclosed above may be assembled on a wafer prior to severing theindividual chips from the wafer. Thus, the foregoing description of thepreferred embodiments should be taken by way of illustration rather thanby way of limitation of the invention set forth in the claims.

[0087] As these and other variations and combinations of the featuresdiscussed above can be utilized without departing from the presentinvention as defined by the claims, the foregoing description of thepreferred embodiments should be taken by way of illustration rather thanby way of limitation of the invention set forth in the claims.

1. A microelectronic assembly comprising: a microelectronic elementhaving a first surface including a central region and a peripheralregion surrounding said central region, said microelectronic elementincluding a plurality of contacts disposed in said central region; acompliant layer overlying said peripheral region of said first surface,said compliant layer having a bottom surface facing toward said firstsurface of said microelectronic element, a top surface facing upwardlyaway from the first surface of said microelectronic element and one ormore edge surfaces extending between said top and bottom surfaces ofsaid compliant layer; and a plurality of flexible bond ribbons disposedover said compliant layer so that said bond ribbons extend over said topsurface and one or more of said edge surfaces and said bond ribbonselectrically connect said contacts to conductive terminals overlying thetop surface of said compliant layer.